Variable attenuator having low minimum insertion loss



Jn. 9, 1968 H. B. lsAAcsoN 3,363,201

VARIABLE ATTENUATOR HAVING LOW MINIMUM INSERTION LOSS Jan. 9, 1968 H. B. lsAAcsoN 3,363,201

VARIABLE ATTENUATOR HAVING LOW MINIMUM INSERTION LOSS Filed March 25, 1965 5 Sheets-Sheet 2 i e T 1o m L 22 3 9 12 13 5 i I /18 7 2*# 6 FIG 4c CM1@ s v -44 d, 3o 3 c1 5 14/ l-J J- lz Jan. 9, 1968 H. B. lsAAcsoN VARIABLE ATTENUATOR HAVING LOW MINIMUM INSERTION LOSS 3 Sheets-Sheet 5- Fled March 25, 1965 N- NN m t f NI! mv rates This invention relates to variable directional couplers and more particularly to variable directional couplers which have insertion losses of less than 3 db when used as variable attenuators.

Variable directional couplers are widely used as power dividers and attenuators. One of the principal difficulties with these devices when used as a variable attenuator is the fact that insertion losses of db and over can be eX- pected when using directional couplers which have a secondary which is moveable with respect to the primary of the directional coupler. This high loss results from discontinuities introduced by reducing the physical spacing between conductors which changes the electrical capacitance and the impedance seen by the incoming energy. Extraordinary measures which incorporate a number of 3 db couplers and cut-off attenuators provide minimum insertion losses in the order of 1 db but such measures require the use of special techniques and are bulky and expensive. Such special techniques often require switching at some point in an attenuation range to a device which operates on a different principle from the directional coupler which provides the bulk of the attenuation, and may present impedance matching, VSWR and linearity problems. Because of these impediments, the apparatus utilizing the directional coupler as an attenuator has its ultimate performance degraded and interpretation of any result is consequently made more diicult. A need, therefore, exists for a variable directional coupler operable as a variable attenuator which has a low insertion loss in the order of l db, which has good impedance and VSWR characteristics and which is capable of being varied lineraly from a maximum to a minimum value of attenuation. Such a device should also be inexpensive, rugged, low in weight and of minimum volume.

It is, therefore, an object of this invention to provide a variable directional coupler for use as a variable attenuator which is superior to prior art devices.

Another object is to provide a variable attenuator which has a minimum insertion loss of less than l db.

Another object is to provide a variable attenuator which is linearly variable from a maximum to a minimum value of insertion loss, the latter being less than 1 db.

Another object is to provide a variable attenuator which has good impedance, and VSWR characteristics.

Another object is to provide a variable attenuator which, for a given setting, has a value of attenuation which is constant over an octave bandwidth.

Still another object is to provide a variable attenuator which introduces minimum values of phase shift with changes in the value of attenuation.

A further object is to provide a variable attenuator which is light weight, inexpensive and rugged.

Another object is to provide a variable attenuator which has an adjustable minimum insertion loss.

A feature of this invention is the utilization of a variable attenuator having first and second transmission lines which are adapted to propagate radio frequency energy therealong and disposed in parallel spaced relationship with each other. Also utilized are means for varying the spacing between the transmission lines to vary the transfer of energy therebetween and means co-acting with the varying means to further vary the spacing of a portion of each of said transmisison lines to obtain a further variation in the transfer of the radio frequency energy.

` are Another feature is the utilization of first and second transmission lines which propagate radio frequency energy therealong in conjunction with means coupled to at least one of the transmission lines for applying relative motion therebetween. Also utilized are means for applying further relative motion between portions of said transmission line such that certain of the portions are in iixed space relationship and other of said portions are electrically short-circuited when said relative motion is terminated.

Still another feature is the utilization of primary and secondary transmission lines each of which has rst and second portions which are adapted for motion relative to each other. Means for translating the primary and secondary transmission lines in a given direction are also utilized and, means for applying other translatory motionto one of the portions of each of said transmission lines in a direction opposite to said given direction to obtain a minimum value of insertion loss is utilized.

Still another feature of this invention is the utilization of a first and second radio frequency transmission lines terminated in their characteristic impedance which are disposed in moveable spaced relationship with one another to variably transfer radio frequency energy between the transmission lines. Also utilized are means connected to each of the transmission lines to vary the electrical capacity and physical spacing between portions of the same transmission line and between a portion of the rst and second transmission lines.

The foregoing and other objects and vfeatures of this invention will become more apparent when taken in conjunction with the following specification and accompanying drawings in which:

FIG. 1 is an isometric view of a preferred embodiment of this invention showing the attenuator in the condition of maximum insertion loss.

FIG. 2 is an exploded isometric view of identical fixed and moveable segments of a block assembly shown in FIG. 1 showing the interleaving arrangement of the segments.

FIG. 3 is an isometric view of a preferred embodiment of this invention showing the attenuator in the condition of minimum insertion loss.

FIG. 4A is a schematic drawing of the attenuator of FIG. l indicating the RF energy paths and the mechanical adjustment of the capacitive impedances involved.

FIG. 4B is a schematic drawing of another embodiment of the present invention showing the RF energy flow paths and the capacitive impedances involved.

FIG. 4C is a schematic drawing of the embodiment shown in FIG. 4B which indicates the physical juxtaposition of the elements of the attenuator and the means for obtaining the desired mechanical motions.

Referring now to FIG. 1, there is shown an isometric view of a preferred embodiment of a variable attenuator according to the present invention. Block assembly 1 is made up of two identical interleaved segments 2, 3. Block assembly 4, is likewise made up of identical interleaved segments 5, 5. Block assembly 1 is adapted to be moved rectilinearly with respect to block assembly 4. Block assemblies 1, 4, each have strip conductors 7, .8, connected to their respective block assemblies. Strip conductor 7 is made up of separabie portions or segments 9 and 1li and strip conductor 8, is similarly made up of separable portions or segments 11 and 12. Portion 9 is attached to interleaved segment 3 by means of conductive pin 13 wi ich connects'to a coaxial cable 14 which, in turn, is carried through segment 3 via hole 15 and Teflon bushing 16. At this point, a distinction should be made between the interleaved segments of the block assemblies 1 and d. interleaved segments 3 and S of assemblies 1, 4, respectively, can be characterized as fixed segments while interleaved segments 2, 6 of 'assemblies l, 4, respectively, can be characterized as moveable segments. The moveable Y segments 2, 6, are moveable relative to the xed segments 3, 5. These latter segments are each attached to back plates 17, 1S, respectively, by means of pins 19, 20, respectively. Pins 19, may be any well known means of mechanically interconnecting members, such as screws. Portion 10 of strip conductor 7 is electrically and mechanically connected to moveable segment 2 by means of conductive pin 21 which connects to coaxial cable 22 which is carried through segment 2 4by means of hole 23 and Teon bushing 24.

trip conductor support members 25, 26 are connected to the extremities of strip conductor portions 9, 10, respectively, at one end thereof, while the other end of members 25, 26, is connected to fixed segment 3 and movable segment 2, respectively. An aperture 27 in portion 10 permits relative motion between member 25 and portion 10. Support members 25, 26 are made of Teflon to provide an insulating support between the segments 2, 3 and portions 9, 10 of conductor 7. Besides acting as insulators, the low friction characteristic of Teflon permits easy translation of support through aperture 27. Finally, to complete block assembly 1, actuating rod 28 is attached to moveable segment 2 so that it clears the extremity of conductor portion 10 and springs 29, 3@ are interposed between back plate 17 and moveable segment 2 such that the springs 29, 30 will be placed in compression when actuation rod 28 is engaged by fixed block segment 5 of block assembly 4 when block assembly 1 is moved toward block assembly 4.

Block assembly 4 is constructed similarly, as has already been indicated. Thus, coaxial cables 31, 32, pass through back plate 18; through segments 5, 6, respectively, by way of holes 33, 34, respectively, and their associated Teflon bushings 35, 36, respectively. Conductive pins 37, 38 electrically and mechanically couple coaxial cables 31, 32 to conductor portions 11, i2, respectively. As described in connection with block assembly 1, conductor support members 39, 4t) on block assembly 4 are similar in every way to conductor support members 25, 26. Support member 40 passes through an aperture 41 (see FIG. 3) in strip conductor portion 12 and is fixed rigidly to the extremity of conductor portion 11, Actuating rod 42 clears the extremity of strip conductor portion 9 and is engaged by lixed block segment 3, as block assembly 1, is rectilinearly moved toward block assembly 4. Springs 43, 44 are disposed between back plate 18 and moveable segment 6 such that the springs are placed in compression by the motion of iixed block segment 3 against actuating rod 42.

Block assemblies 1, 4 are completely surrounded by spring contact material 45 which is aiixed to fixed block segments 3, 5 by means of screws 46. Spring contact material 45 is a conductive metal of copper foil, for instance, which insures good contact with the metal walls of a surrounding chamber 47 into which both assemblies 1, 4 are placed. A cover plate (not shown) encloses both assemblies 1, 4 and contacts spring Contact material 45 to provide good electrical contact with the walls of chamber 47 to prevent the leakage of radio frequency energy past the block assemblies 1, 4. A drive rod 48 passes through a wall of chamber 47 and is aiixed to back plate 17 to provide for the rectilinear movement of block assembly 1. The simplified drive arrangement of FIG. 1 can, of course, be replaced by more elaborate arrange* ments as is apparent to those skilled in the mechanical arts. Back plate 18 is tixed rigidly to chamber 47 and the only motion allowed in block assembly 4 is restricted motion of moveable block segmentk relative to iixed segment 5 when actuated by actuating rod 28.

A feature of this invention is the utilization of variable length actuating rods 23, 42 which are adapted to be screwed into threaded holes 42a, for instance, in FlG. V2

. to adjust the closeness to which conductor segments 9,

11 may approach each other. The closeness, of course, determines the minimum value of attenuation obtainable as will be seen in the discussion hereinbelow.

Actual contact of conductorV portions is prevented by a strip of adhesive backed Teflon tape 49 which is shown applied to one of the opposing faces of strip conductor positions 9 and 11. In like manner, conductor portion 10 is separated from conductor portion 9 by a strip of Teflon tape 50 and conductor portion 12 is separated from portion 11 by a similar strip of tape 51. The reason for the separation of the various conductor portions will be discussed when the electrical effects utilized in the present invention are considered.

Referring now to FIGS. 1 and 2, FIG. 2 shows a block segment several of which are utilized to form block assemblies 1, 4.

The upper :block segment shown in FIG. 2 is shown in FIG. l as fixed block segment 5 of block assembly 4 while the lower block segment is shown in FIG. 1 as moveable block segment 6 of block assembly 4. Each block segment is identical with the other and is machined to permit the interleaving of the segments to form a block assembly 4. Identical length sections, x, y, z, of block segment 5 are similarly marked on block segment 6. To assemble segment 5, 6, segment 6 is interleaved with segment 5 along the path of the dotted arrows in FIG. 2. Thus, section z of segment 6 mates with section y of segment 5 and section y of segment 6 mates with section z of segment 5. In this manner, a block assembly 4, having a uniform thickness t and a length l equal to four times the length of one of the sections x, y, z, 4x, for instance is provided. Block assembly 4, if all the sections x, y, z mated perfectly, would be a completely solid block. Perfect mating, however, is not allowed to take place. This is accomplished by making the sum of the widths of section y and that portion of sectionr z of thickness t less than one-half the width w of each segment. Thus, segments 5 and `6 can be translated along Van axis parallel to the width Vof the segments in a back and forth motion so that the overall width of block assemblycan be varied from a width of w to almost 2w depending on the widths chosen for sections y and those portions of sections z of thickness t.

From FIG. 2, it should be clear that a single repetitive operation can be utilized to mass produce the block segments which can be made of machined aluminum, for instance, or other conductive material. At least a portion of the surface of the block segments (see shading in FIG. 2) should be conductive since that portion acts as a ground plane for the strip conductors 7, S which are mounted in close proximity to the block segments. Block segments 5, 6 in FIG. 2 could be, therefore, plastic or other useful material having metallized portions as shown by the shading in FIG. 2. The interior of holes 33, 34 which carry coaxial conductors 31, 32 through block segments 5 and 6 alternatively could be provided with metallic inserts to form the outer conductor of a coaxial line which carries RF energy to conductor portions 11, 12. Alternatively, holes .33, 34 can be adapted to contain terminations Z0 of resistive lm which are well known to those skilled in the microwave art. By this means, only the input and output RF cables '14 and 31, respectively Y of FIG. 1 need extendfrom housing or chamber 47 providing for a compact, highly rugged variable attenuator. It is interesting to note that the structural arrangement and juxtaposition of block segments permits holes 33 and 34 to be drilled through section x as shown in FIG. 2 and holes 39a, 40a and 42a are disposed to receive support members 39, 40, and actuating rod 42, respectively. Since the same holes have the same positions on each of block segments 5, 6 a single drilling orV casting operation is all that is required in the mass production of these segments.

Referring now to FIG. 3, the variable attenuator of y.

FIG. 1 is shown in the position of minimum Vinsertion' loss, i.e. in the position of closest proximity of portions 9, 11 of strip conductors 7, `8, respectively. In this condition, push rod 48 has translated block assembly 1 in the direction of block assembly 4. As the assemblies 1, 4 approach each other, actuating rods 2S and 42 which are oi equal length butt up against fixed block segments 5 and 3, respectively. Further, translation by push rod 48 causes fixed segment 3 to push against actuating rod 42 causing movable block segment 6 to be moved against springs 43, 44 thereby causing strip conductor portion 12 to separate from strip conductor portion 11. In like manner, the further translation by push rod 48 causes fixed segment 5 to push against actuating rod 28 which, in turn, causes movable segment 2 to be moved against springs 29, 30 in a direction opposite to the direction of motion of push rod 4S. The movement of segment 2 causes strip conductor portion to move away from portion 9 at the same relative rate as portion 12 moves away from portion 11. At the point of minimum insertion loss as shown in FIG. 3, conductor portions 9, 11 butt against one another separated only by Teiion strip 49 which acts as a dielectric between portions 9 and 11. The condition of minimum insertion loss is the maximum point of electrical and mechanical decoupling of the conductor portions of strip conductors 7, 8. Thus, the rectilinear movement of block assemblies 1, 4 increases the mechanical and electrical coupling of conductor 7, 8 until actuating rods 28, 42 contact their respective block segments; at which point portions of the same strip conductors 7, S are electrically and mechanically decoupled while one portion of the conductor 7 approaches a point of maximum electrical and mechanical coupling with another portion of the conductor 8.

The foregoing description has discussed principally the mechanical considerations of the attenuator disclosed. A discussion of the electrical considerations involved will now be provided in conjunction with FIGS, 1, 3, 4A and 4B.

In FIG. l, under the usual conditions where strip conductors 7, 8 would be integral members having no separable portions, ports 2 and 3 would be terminated in the characteristic impedance of the transmission lines 7, 8. RF energy introduced at port 1, in such an arrangement, would be coupled to port 4; the operation being expected and in accordance with usual coaxial or stripline directional coupler operation. At the point of closest approach of strip conductors, the power output at port 4 would, at best, be one-half the input power at port 1. Thus, under the best circumstances the loss in the 3 db coupler, as such devices are termed, is theoretically onehalf the input power. In actual operation, more often than not the power transferred is down in excess of 5 db. Also, the VSWR at the minimum insertion ioss setting is often no better than 2 to 1.

The technique of the present invention, however, provides a variable attenuator which has a substantially constant value of attenuation for a given setting of push rod 43 over an octave bandwidth.

In the arrangement of FIGS. 1 and 3, ports 2 and 4 are terminated in the characteristic impedance (Z0) of strip conductors 7, 8. The normally isolated port 3 utilized as the output of the variable attenuator. In operation, when block assemblies 1, 4 are furthest apart, the insertion loss is very high (9C-120 db). As the block assemblies 1, 4 approach each other, the attenuation decreases gradually in the usual fashion. It was unexpected that power would be transferred without a high VSWR at port 1 at low values of attenuation .because it was expected that large discontinuities would be introduced upon separation of the conductor portions 9, 11, 16, 12. Any discontinuities which were expected by the separation of the conductor portions 9, 19, 11, 12 are apparently matched out by the approach of conductor portion 9 to conductor portion 11. A careful consideration of FIGS. l and 3 shows that at high values of insertion loss, the

capacitors formed by conductor portions 9, 10 and Teflon strip 50 and by conductor portions 11, 12 and Teflon strip 51 present a negligible impedance to the RF energy being transferred between strip conductors 7, 8. In FIG. 3, however, as conductor portions 11, 12 and 9, 1t? separate the capacitive impedance becomes considerable and the RF energy on strip conductors 7, 8 is presented with a gradually increasing impedance until, at the point of minimum attenuation, an open circuit exists between conductor portions 9, 1i) and between conductors 11, 12. Also, as conductor portions 9, 11 approach each other, RF energy being propagated along strip conductors 7, S encounters a capacitive impedance which varies from an open circuit value to a substantial short circuit at radio frequencies at the point of closest approach. The capacitive reactance at the point of minimum insertion loss is governed by the equation:

(7:.2256; where e=dielectric constant of Teiion. A=area in sq. in. f=thickness of dielectric in inches.

Thus, as one value of capacitive reactance decreases, the values of the other capacitive reactances increase, permitting, at all values of frequency, the smooth transfer of power between ports 1 and 3 by the linear actuation of push rod 48. The schematic diagram of FIG. 4A with the conductors 7, 8 in the same juxtaposition as shown in FIGS. l, 3 should clarify the manner in which the capacitive reactances vary. Thus, in FIG. 4A the lcapacitance C1 between conductor portions 9, 11 gradually decreases as these conductors approach each other until a substantial short circuit at radio frequencies exists transferring power directly from port 1 to port 3. Capacitors C2 and C3 gradually increase from a substantially short circuit condition to an open circuit condition as capacitor C1 varies in the opposite manner, providing an open circuit, at the point of minimum insertion loss, between ports 1 and 2 and between ports 3 and 4. These lump circuit reactances ico-act in such a way as to provide a constant power output at a given setting of push rod 48 over a wide range of frequencies at port 3. This results from the fact that the summation of the curves of coupling and directivity each of which is not perfectly fiat over the usual frequency ranges, have curvatures in opposite directions which when scanned provide a tlat or constant attenuation over the whole frequency range.

FIG. 4B shows an alternative arrangement of the positions of conductor portions 9, 10, 11, 12. In this arrangement conductor portions 9, 12, butt against each other in the position of minimum insertion loss rather than portions 9, 11 as shown in FIG. 4A. Note, however, that capacitors C1, C2 and C3 are still present and can be linearly varied in a manner similar to that shown in FIGS. 1, 3. Rather than having a moveable block segment oppose a iixed block segment as shown in FIGS. l, 3 in the arrangement of FIG. 4B, a fixed block segment would oppose a fixed block segment and a moveable block sevment would oppose a moveable block segment. An actuating rod attached to a moveable segment would actuate an opposing moveable segment and both moveable segments would be forced back against their associated springs, FIG. 4C shows a schematic arrangement of the device ot 4B including schematic representations of the fixed and moveable segments. All the major portions shown have the same reference numbers as the related parts shown in FIGS. 1, 3. Experimental results have indicated that the arrangement of FIG. 4C, while it represents a substantial improvement over prior' art devices, the minimum insertion loss is somewhat in excess of the l db or less obtainable with the preferred embodiment shown herein in FIGS. 1 and 3. In connection with FIGS. 4B, 4C, it should be noted that the po`wer output is taken from port 4 as is usual in devices of this type and, therefore, different from the device of FIG. 4A wherein the power output is obtained from port 3. In the arrangement of FIG. 4B, port 3 is isolated by virtue of the open circuit between conductor portions 11 and 12.

While the device described herein above has been described as a variable attenuator, it can also be characterized as being a variable `directional coupler and as such would in no way be changed from the device characterized as a variable attenuator. The arrangements described herein `can be utilized in conjunction with any TEM mode microwave device and can operate in any frequency range capable of being supported by coaxial transmission lines.

The devices described hereinabove have been described as unitary structures capable of operation over at least anoctave bandwidth, for instance, but it should be apparent that a plurality of the devices described operating in overlapping frequency ranges can be cascaded to provide a minimum insertion loss of l db or less over a frequency range greater than an octave.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A variable attenuator comprising first and second transmission lines adapted to propagate radio frequency energy therealong disposed in spaced relationship each of said transmission lines comprising discrete portions disposed in overlapping relationship along a length sufficient to provide substantial coupling between said lines, at least one of said portions being moveable relative to another of said portions, means for varying the spacing between said transmission lines to vary the transfer of said energy therebetween and means coacting with said last mentioned means to vary the spacing of said portions of each of said transmission lines to obtain a further variation in the transfer of said energy.

2. A variable attenuator according to claim 1 wherein said means for varying the spacing between said transmission lines includes a fixed and moveable assembly to which are mounted said transmission lines and means for translating said moveable assembly relative to said fixed assembly mechanically connected to said moveable assembly.

3. A variable attenuator comprising first and second transmission lines adapted to propagate radio frequency energy therealong disposed in spaced relationship, means for varying the spacing between said transmission lines to vary the transfer of energy therebetween and means coacting with said last mentioned means to further vary the spacing of a portion of each of said transmission lines to obtain a further variation in the transfer of said energy including a moveable segment connected to each said portion of said transmission lines and a fixed segment connected to a portion of each of said transmission lines other than said portion, said fixed and moveable segments being in interleaved moving relationship such that said portion and a portion other than said portion of each of said transmission lines form a continuous transmission line when said segments are mated and means connected to at least one of said segments for applying relative motion between said fixed and moveable segments to separate said portions and portions other than said portions of each of said transmission lines .and join one of the mentioned portions of each of said transmission lines to form a new 4continuous transmission line.

4. A variable attenuator according to claim 3 further including means for restoring said fixed and moveable segments to a mated condition upon separation of said one, the mentioned portions of each of said transmission lines.

5. A variable attenuator according to claim 4 wherein said means for restoring includes at least a compressible member and back plates attached to said fixed segments and coextensive with said interleaved xed and moveable segments said compressible member being interposed between said back plates and said moveable segments to remate said xed and moveable segments.

6. A variable attenuator according to claim 3 wherein Y said means connected to at least one of said segments for applying relative motion between said fixed and moveable segments includes an actuating rod extending from each` of said moveable segments adapted to engage an oppositely disposed fixed segment by motion of said varying means.

7. A variable attenuator according to claim 3 wherein said means connected to at least one of said segments for applying relative motion between said fixed and moveable segments includes an actuating rod of variable length extending from each of said moveable segments adapted to engage an oppositely disposed xed segment by motion of said varying means and means to vary the length of said actuating rod to vary the value of minimum insertion loss.

8. A variable attenuator according to claim 7 wherein said means to vary the length of said actuating rod includes a tapped hole in said moveable segments adapted to mate with a threaded portion on said actuation'rod.

9. A variable attenuator according to claim 3 wherein said means connected to at least one of said segments for applying relative motion between said fixed and moveable segments includes an actuating rod extending from a moveable segment adapted to engage an oppositely disposed moveable segment by motion of said varying means.

10. A variable attenuator according to claim 3 wherein said means connected to at least one of said segments for applying relative motion between said fixed and moveable segments includes an actuating rod of variable length extending from a moveable segment adapted to engage an oppositely disposed moveable segment by motion of said V varying means and means to Vary the length of said actuating rod to vary the value of minimum insertion loss.

11. A variable attenuator according to claim 10 wherein said means to vary the length of said actuating rod includes a tapped hole in said moveable segment adapted to mate with a threaded portion on said actuating rod.

12. A variable attenuator comprising primary and secondary transmission lines adapted to transmit radio frequency energy therealong, each of said transmission lines having first and second discrete portions disposed in overlapping relationship along a length sufficient to provide substantial coupling between said lines adapted for motion relative to each other, means for translating said primary and secondary transmission line in a given direction and, means for applying further translatory motion to one of said portions of each of said transmission lines in a direction opposite to said given direction to obtaina minimum value of insertion loss.

13. A radio frequency attenuator comprising a first radio frequency transmission line comprising discrete overlapping portions adapted to propagate radio frequency energy therealong and terminate in its characteristic impedance, a second radio frequency transmission line comprising discrete overlapping portions adapted to propagate radio frequency energy therealong terminated at one end thereof in its characteristic impedance disposed in moveable spaced relationship'with said first radio frequency transmission line to variably transfer radio frequency energy between said transmission lines said discrete portions being disposed in overlapping relationship along a length sufl'icient to provide substantial coupling between said lines and, means connected to each of transmission lines to vary the electrical capacity and physical spacing between said discrete portions of the same trans-V mission line and between said rst and second transmission lines.

14. A radio frequency attenuator comprising a first radio frequency transmission line comprising discrete overlapping portions adapted to propagate radio frequency energy therealong and terminated in its characteristic impedance, a second radio frequency transmission line comprising discrete overlapping portions adapted to propagate radio frequency energy therealong terminated at one end thereof in its characteristic impedance disposed in moveable spaced relationship with said first radio frequency transmission line to variably transfer radio frequency energy between said transmission lines, said discrete portions being disposed in overlapping relationship along a length sufcient to provide substantial coupling between said lines and, means connected to each of said transmission UNITED STATES PATENTS 3,166,723 l/l965 Bock et al 333-10 FOREIGN PATENTS 514,235 6/ 1965 Canada.

HERMAN KARL SAALBACH, Primary Examiner.

M. NUSSBAUM, Assistant Examiner. 

1. A VARIABLE ATTENUATOR COMPRISING FIRST AND SECOND TRANSMISSION LINES ADAPTER TO PROPAGATE RADIO FREQUENCY ENERGY THEREALONG DISPOSED IN SPACED RELATIONSHIP EACH OF SAID TRANSMISSION LINES COMPRISING DISCRETE PORTIONS DISPODED IN OVERLAPPING RELATIONSHIP ALONG A LENGTH SUFFICIENT TO PROVIDE SUBSTANTIAL COUPLING BETWEEN SAID LINES, AT LEAST ONE OF SAID PORTIONS BEING MOVEABLE RELATIVE TO ANOTHER OF SAID PORTIONS, MEANS FOR VARYING THE SPACING BETWEEN SAID TRANSMISSION LINES TO VARY THE TRANSFER OF SAID ENERGY THEREBETWEEN AND MEANS COACTING WITH SAID 